Separators currently used in lead acid (or storage) batteries are microporous films of sheet material that prevent shorting between neighbouring electrode plates of opposite polarities and plate material from falling out, but on account of their porous structure permit ionic current flow in the electrolyte. Separators of this kind are known from, for example, U.S. Pat. No. 3,351,495, U.S. Pat. No. 4,927,722, U.S. Pat. No. 5,776,630 and WO 01/13 442 each is incorporated herein by reference. Typical polymers these separators are made from include polyolefins such as high molecular weight polyethylene (e.g., ultra high molecular weight polyethylene, UHMWPE). Such separators are normally provided on at least one side with longitudinal ribs that are intended to prevent direct contact of the sheet material with the positive electrode plate and maintain the spacing between the opposing electrodes. These ribs also lend the separator a certain rigidity in the longitudinal direction. Longitudinal ribs of this kind may also, as described in U.S. Pat. No. 5,679,479 and U.S. Pat. No. 5,789,103 both incorporated herein by reference, consist of a plurality of individual stamped embossments that form a rib structure of alternating ridges and furrows.
Separators are usually manufactured by extrusion of a thermoplastic into a film that is then rolled into a sheet material with the prescribed ribs, after which the pore former, such as mineral oil, is extracted and the sheet material so formed is wound into rolls. This sheet material is later drawn off the roll and cut into strips of the desired width. These strips are cut to the desired length and then folded over either a positive or a negative electrode plate to form an envelope, the two peripheral regions of which can be joined by, for example, heat sealing, pressure welding or other processes that are known per se. Electrode plates are then assembled into groups for a storage battery, plates in separator envelopes alternating with plates of opposite polarity without envelopes. In general, only electrode plates of a single polarity are placed in envelope separators; in special cases, however, electrode plates of both polarities can be placed in envelope separators. The electrode plates within a group are now aligned and then joined together. The alignment of the electrode plates may result in individual electrode plates being pushed to a greater or lesser extent into one or the other peripheral region of an envelope separator. Because the electrode plates frequently acquire pointed tips or sharp edges depending on the manufacturing process, this displacement can cause a point or edge of an electrode plate to puncture the separator material, which in turn can result in shorting with the neighbouring electrode. This is especially the case when the electrode plates used consist of a grid of, for example, expanded or stamped metal into which the actual active material is incorporated, as described for example in EP-A-0 994 518. In such cases it may occur that the expanded material is not cut precisely at the nodes, so that individual grid wires project from the electrode plates, bend slightly on alignment of the electrode plates and puncture the sheet material of the separator.
To avoid, or at least reduce, the risk of puncture by the points, edges or grid wires of the electrode plates, it has been proposed that the peripheral region be designed as a plurality of parallel, concave furrows interspersing a plurality of similarly formed small “mini-ribs” (EP-A-0 899 801, JP 2000-182593, U.S. Pat. No. 6,410,183 each is incorporated herein by reference) and that the ribs between the furrows be broader and with a flat upper surface (WO 00/63983 incorporated herein by reference). It has been shown, however, that in unfavourable cases it is still possible for individual envelope separators to be punctured during alignment of the electrode plates, with the associated risk of shorting. It has also been proposed to design the mini-ribs in the form of a triangle projecting from the level of the sheet material, with the base of the triangle on the sheet material, one side of the triangle facing the inner region and the other side facing the periphery, the side facing the inner region being longer than that facing the periphery.
However, it has been found for the prior art separators that many punctures still occur in the peripheral region with separators having mini-rib configurations.